On Tuesday, the House Oversight and Government Affairs Committee accused IRS Commissioner Danny Werfel of stonewalling its investigation into the agency’s targeting of conservative and Tea Party groups that applied for tax-exempt status.

According to a letter the committee chair, Rep. Darrell Issa (R-CA), wrote to Werfel, the IRS has “attempted to delay, frustrate, impede and obstruct” the investigation in a “systematic manner.”

“The actions of the IRS under your leadership have made clear to the committee that the agency has no intention of complying completely or promptly with the committee’s oversight efforts,” Issa wrote in the letter. The letter also warns that obstructing a congressional investigation “is a crime.”

Sources told FoxNews.com that subpoenas may even be issued in the future if the stonewalling continues.

The IRS failed to comply on at least three occasions, according to the letter, since the targeting revelations came to light. The “agency has turned over just 12,000 pages of the roughly 64 million pages it deemed potentially relevant to the investigation.” Issa also said many of the documents turned over to Congress are “completely unintelligible and useless” because of excessive redactions. Further, the IRS has also blocked an employee in the Exempt Organizations Determinations Office from turning over important documents.

House Investigators also discovered that conservatives and Tea Party groups were asked three times as many questions as other groups and were less likely to get approval for tax-exempt status.